1. Field of the Invention
The invention pertains to the fabrication of integrated circuits utilizing metal oxides, such as perovskites and perovskite-like layered superlattice materials, and more particularly to the use of ultraviolet ("UV") radiation in the fabrication of such materials for use in integrated circuits.
2. Description of the Prior Art
Ferroelectric perovskite-like layered superlattice materials are known, and have been reported as phenomenonological curiosities. The term "perovskite-like" usually refers to a number of interconnected oxygen octahedra. A primary cell is typically formed of an oxygen octahedral positioned within a cube that is defined by large A-site metals where the oxygen atoms occupy the planar face centers of the cube and a small B-site element occupies the center of the cube. In some instances, the oxygen octahedra may be preserved in the absence of A-site elements. The reported annealing temperatures for these materials often exceed 1100.degree. C. or even 1300.degree. C., which would preclude their use in many integrated circuits.
Ferroelectric materials can be used in electronic memories. The respective positive and negative polarization states can be used to store information by substituting a ferroelectric material for the dielectric capacitor material of a conventional DRAM capacitor circuit. Still, ferroelectric memory densities are limited by the magnitude of polarization that may be obtained from the ferroelectric material. Prior thin-film ferroelectric materials typically have high polarization fatigue rates that make them unreliable in long term use because the magnitude of polarization decreases with use. Eventually, the control logic that is coupled with known ferroelectric materials will be unable to read the fatigued polarization state of the materials and, therefore, unable to store or retrieve bits of information.
Prior layered superlattice materials typically have high dielectric constants, and can be used as conventional dielectrics. Even so, a high leakage current makes these materials poorly suited for use as a dense dielectric memory because the charged or uncharged state of the dielectric capacitor circuit must be too frequently detected and refreshed. High leakage current and long term reliability currently present a significant obstacle in research toward further densification of integrated circuit memories.
Spin-on liquid deposition processes have been used to make insulators for integrated circuits, such as spin-on glass (SOG), and have also been used for making metal oxides having perovskite structures. See G. M. Vest and S. Singaram, "Synthesis of "Metallo-organic Compounds For MOD Powders and Films", Materials Research Society Symposium Proceedings, Vol. 60, 1986, pp. 35-42, IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, Vol 35, No. 6, November 1988, pp. 711-717, and "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", Materials Research Society Bulletin, October 1989, pp. 48-53. Nevertheless, the quality of the thin films made in these references was too poor for use in integrated circuits.
Ultraviolet radiation has been utilized as an energy source to disassociate hydroxyl bonds in vapor deposition of thin films. See U.S. Pat. No. 5,119,760 issued to two of the present inventors. There has been no indication in the art that the use of UV radiation may be useful in solving the problem of the high leakage current in metal oxide dielectrics such as strontium titanate. Additionally, there has been no indication that UV radiation could be useful in increasing fatigue resistance or the magnitude of polarization obtainable from a metal oxide material having a particular stoichiometric composition.